This invention relates to the correction of distortion introduced by multipath fading in a radio system and, more particularly, to an adaptive equalizer which acts to cancel the distortion so introduced.
The adverse effects of multipath propagation have been studied for some time. Frequency selective fading due to multipath propagation has been tolerated on microwave paths for many years because these older systems employed frequency modulation, which is inherently resistant to the linear amplitude distortion component that predominates in multipath induced distortion. With the introduction and rapidly expanding use of digital modulation in microwave transmission systems the deleterious effects of multipath-induced in-band distortion requires that such distortion be compensated.
The amplitude and delay characteristics which occur as a result of delay in the arrival of a refracted ray as compared to a direct ray are illustrated in an article "An Adaptive Equalizer For Correction Of Multipath Distortion In A 90MB/s PSK System," by Paul R. Hartmann and Eddie W. Allen, IEEE, ICC 79 Record, Vol. I, pp. 5.6.1-5.6.4. The authors analyze a two-ray model, i.e. direct and refracted rays, and note that the bit error rate (BER) during multipath is controlled by intersymbol interference resulting from frequency dependent amplitude and group delay effects. These deleterious effects were separated into distortions which result in threshold degredation and which could be classified as linear amplitude effects (gain slope), linear group delay and parabolic group delay. Based upon a two-ray model, the authors analyzed the importance of these three effects by calculating the amplitude and group delay slopes in a 30 MHz band width resulting from two-ray 10 dB fades at different absolute delays. It was determined from this analysis that for a given delay and fade depth, the effect of amplitude slope on threshold degradation was most pronounced, i.e., it is a primary source of performance degradation during multipath conditions. Once the amplitude slope degradation was removed, the major deleterious effect was a symmetrical null at the center of the passband. The adaptive equalizer, not shown in the article, was designed to correct for amplitude slope by comparing the signal amplitudes at the two ends of the band and using this information to correct the slope. Similar slope correction arrangements have been and are being used in carrier current systems. A null detection circuit was included in the equalizer and operates on the basis of comparison of the energy at the center of the passband with the total energy in the passband.
A theoretical technique employing the power series modeling for compensation of the distortion effects introduced by multipath fading is briefly discussed in an article "Adaptive Equalizer For Digital Microwave System," by Paul Hartmann and Brian Bynum, ICC 80 Record, pp. 8.5.1-8.5.6. The power series technique shown in FIG. 7 of this article shows a theoretical approach to compensation by introducing amplitude slope, parabolic and cubic modifications of the IF band. The manner in which such modifications may be obtained is not disclosed nor is group delay compensation incorporated in the model shown.